The invention relates to a focusing-device for the radiation from a light source, in particular a laser plasma source, having a collector mirror according to the preamble of claim 1 of a type defined more closely.
In micro-lithography, use is frequently made of a light source, for example a laser plasma source, whose light is collected, in virtual or real terms, via a collector mirror at a second focus, and then guided for beam formation into a illuminating system. The collector mirror is heated by the laser plasma source, resulting in corresponding changes in shape which have negative effects on the downstream illuminating system such as, for example, illumination defects, for example telecentring errors, uniformity defects, and this can lead to light losses.
It is known for the purpose of avoiding these disadvantages to cool the collector mirror in order to dissipate the heat produced. Irrespective of the large outlay required for this purpose, because of unavoidable tolerances problems continue to exist, nevertheless, with reference to the imaging accuracy, and these are to be ascribed, inter alia, to a change in position of the second focus. In addition, given high thermal loads, which can vary strongly with time in the case of pulsed operation, the collector mirror cannot be kept entirely at a constant temperature level, and so would necessitate a xe2x80x9cdynamic xe2x80x9d cooling system,
It is the object of the present invention to create a device in the case of which the disadvantages of the prior art are avoided, in particular in the case of which the optical properties of a collector mirror are maintained in an unchanged form even under thermal loading such that no negative effects on the downstream illuminating system occur.
According to the invention, this object is achieved by means of the features claimed in claim 1.
By virtue of the fact that the collector mirror is specifically displaced in the z-direction, that is to say in the direction of the optical axis, and that the collector mirror itself is designed in such a way that the position of the second focus remains unchanged in the event of temperature change, the optical properties of the collector mirror are maintained in an unchanged form even under thermal loading.
It is proposed according to the invention in a first design solution to mount or configure the collector mirror such that its shape is formed in accordance with an isofocal family of curves, for example a family of ellipses, a family of hyperbolas or a family of parabolas. What is meant by an xe2x80x9cisofocalxe2x80x9d family of curves is that the spacing from the source, that is to say from the first focus, to the second focus does not change. Only a family of ellipses will be spoken of below, for the sake of simplification, If an isofocal family of ellipses projects a source into a fixed image of the source. The collector mirror is now shaped under heating in accordance with the isofocal family of ellipses, its optical properties remain constant . This means that it is then no longer necessary to cool the collector mirror, or to keep it at a constant temperature, by means of a high outlay, but that heating is permitted while ensuring, however, that the change in shape of the collector mirror resulting therefrom takes place such that selected optical properties remain unchanged.
For such an isofocal collector mirror, the conic constant K and the semiparameter p=R can be represented to a good approximation by linear functions of the intercept distance between the source and the vertex of the collector mirror.
If, on the other hand, the aim is to avoid a change in magnification, normally negligible per se, owing to a change in or displacement of the collector mirror, the spacing from the source to the imaging plane of the light source must be varied as second solution. This can be performed, for example, actively or else via a passive thermal expansion. The eccentricity "Ugr" or conic constant K must remain constant for such a collector mirror which contains magnification, and the vertex curvature p=R must change linearly. This solution is advantageous in some circumstances for a system with critical illumination, because then the image of the light source remains the same size on the reticule.
By contrast with the solution using the isofocal family of curves, the spacing between the first and the second focus remaining the same, in this alternative solution the beam angle from the collector mirror to the second focus remains the same, the second focus thereby being displaced correspondingly. If the second focus is to remain at the same point, it follows that not only the collector mirror must be moved correspondingly in the z-direction, but also the source or the first focus.
Since collector mirrors are generally subjected to an anisotropic thermal loading, it may be provided in a refinement according to the invention that the collector mirror is provided with inhomogenously distributed cooling devices in such a way that an at least approximately uniform temperature distribution is achieved in the collector mirror. Although this does entail a higher outlay, by comparison with known cooling devices, however, this outlay can be kept markedly lower, because there is no need to carry out an entire cooling of the collector mirror, but only to ensure a largely uniform temperature distribution.